Several enhanced oil recovery (“EOR”) methods have been demonstrated to have the ability to produce oil from reserves that would otherwise be inaccessible due to their depth below ground-level. Among these methods, thermal EOR techniques in particular have been proven to be effective for oil recovery; these processes often use once-through steam generators (“OTSGs”) to produce the high pressure injection stream necessary for the process.
OTSGs typically produce a steam that is between 50% and 90% quality and, in some cases, requires the liquid phase from the vapor phase prior to injection to the reservoir. In such processes, the blowdown stream is often unfit for recycle due to elevated content of dissolved solids that have been concentrated between 2× and 10× in the steam generator. This OTSG blowdown stream is cooled, and a falling film evaporator is used to recycle a fraction of this water to the OTSG feed using mechanical vapor compression (“MVC”) process. This technology is able to recover the water and reduce or eliminate the volumetric flow of waste water for disposal.
One disadvantage of the conventional method described above is the high specific energy consumption required for the MVC process. The operating costs associated with this amount of vapor compression are high and represent a prohibitive barrier to using such a process for water recovery. OTSG blowdown recovery processes commonly are only installed when there is a strong regulatory incentive, either in the form of water disposal limitation or make-up water restriction.
Another disadvantage of existing technology is that downstream evaporative zero liquid discharge (ZLD) technology typically also requires MVC design to complete the recovery/recycle of the OTSG blowdown stream. An MVC process for the ZLD evaporator represents an additional parasitic power consumption on the overall process and is a drawback to the process that could otherwise be used for complete water recycle and zero liquid discharge.
A further disadvantage to the conventional process is that the system requires a cooling system to reduce the temperature of the OTSG blowdown stream to near-boiling temperature (typically 90-98° C.) prior to being fed to the evaporator system. Historically, the OTSG blowdown is cooled by two or more heat exchangers arranged in series. The first cooling heat exchanger is conventionally designed to transfer heat to the steam generator feed water stream and is called the Boiler Feed Water (“BFW”) Preheater. This is a positive use of energy since the BFW Preheater will reduce the utility heating requirement, often by another BFW Preheater which uses a hot glycol stream to finish the BFW preheating. The OTSG Blowdown temperature after being partially cooled by the BFW Preheater will have a temperature in excess of 140° C. and is further cooled by a Trim Cooler (with cooling glycol) to approximately 90° C. Refer to FIG. 9. The requirement of the extra heat exchanger(s) adds cost and additional space or footprint is also consumed.